1. Field of the Invention
The present invention relates generally to a light emitting diode composed of a compound semiconductor containing aluminum, and more particularly, to a light emitting diode having a structure which is superior in humidity resistance and has a long life.
2. Description of the Prior Art
In a light emitting diode composed of a compound semiconductor containing aluminum having a gallium aluminum arsenic (GaAlAs) layer and the like, an insulating layer (coating film) composed of silicon nitride has been conventionally formed on the surface of its device so as to increase humidity resistance and improve light extraction efficiency, as disclosed in, for example, Japanese Patent Laid-Open Gazette No. 226181/1989. Such a structure is formed as the measures of the problem peculiar to a compound semiconductor containing aluminum that the oxide or the hydroxide of aluminum is liable to be mainly formed on the upper surface of the device, thereby to form a light absorbing layer as well as to degrade the device and decrease the luminance under humidity conditions, so that the life of the device is significantly reduced.
In such a structure, however, the above described insulating layer does not sufficiently adhere to the upper surface of the above described device or is damaged due to, for example, heat treatment for ohmic contact of electrodes and the impact in the subsequent process of assembling a lamp and the like. Consequently, such circumstances have often occur that the above described oxide or hydroxide of aluminum is expanded, irrespective of the presence of the insulating layer, on the surface of the device centered around a portion where the insulating layer does not sufficiently adhere or is damaged, so that the life of the device is reduced.
Particularly in a structure in which stepped portions are formed in the peripheral part of the above described device by mesa etching, such a phenomenon is, in many cases, centered around edge parts of the stepped portions.